Increased sophistication in aircraft has resulted in so-called "fly-by-wire" aircraft wherein the controls operated by the pilot and the control surfaces responsive to such controls are not directly mechanically coupled. Rather, the same may be coupled by electrical or hydraulic systems. That in turn requires that electrical or hydraulic energy be available at all times or else the link between the controls and the control surfaces is lost.
Consequently, such aircraft require advanced airborne secondary power systems that are operable to provide the requisite electrical or hydraulic energy in a very short period of time following failure of a primary power system. For example, the same must be capable of virtually instant start up from an inactive condition, whether at sea level or extremely high altitudes in excess of 50,000 feet. While this requirement does not present a great deal of difficulty at many altitudes, at the high altitudes, where the air is the least dense, starting of a secondary power system, which is typically turbine driven, such that energy therefrom will be available to operate control surfaces in response to control commands within the requisite elapsed time of two or three seconds poses special difficulties.
One prior art proposal includes the use of hydrazine which is stored on board the aircraft. The hydrazine is applied to a catalytic chamber wherein it decomposes and generates gas under pressure sufficient to drive a turbine wheel and accelerate the same to the point where turbine operation can become self-sustaining at high altitudes. While this works well, storage of unstable material such as hydrazine aboard aircraft is not desirable and the same requires special handling and may not be readily available for use at remote sites.
To obtain the benefits of a stored energy system while avoiding the difficulties that accompany the use of material such as hydrazine, it has been proposed to provide an auxiliary combustor in which the usual fuel employed to power the aircraft is combusted with a stored oxidant which may be air, oxygen enriched air or even, in some instances, molecular oxygen. Because of the need for rapid start up of such systems, substantial gas flows from the auxiliary combustor may be required to achieve the desired acceleration of the turbine wheel so that the secondary system may be generating the required amount of electrical or hydraulic energy within two or three seconds. Kenney et al, in U.S. Pat. No. 2,873,577 issued Feb. 17, 1959 recognized that the flow rate of a combustible mixture in a combustor of a gas driven turbine considerably affects the ignitability of the mixture. And of course, it has long been recognized that flame stability in a combustor of a gas driven turbine is likewise affected by flow rates. Further, since the flow rate of the combustible mixture through the combustor will affect the flow rate of the products of combustion from the combustor to the turbine, the flow rate factor must be examined.
If one concerns oneself only with the necessary flow rate required to rapidly accelerate the turbine wheel, one may find oneself in the position of having a system wherein the flow rate of the combustible mixture to the combustor is sufficiently high that starting reliability and/or combustion stability are sacrificed. Conversely, if one concerns oneself primarily with starting reliability and combustion stability, the required flow rate for acceleration of the turbine wheel may not be obtained or, if obtained, may require the use of a relatively large combustor. A large combustor will necessarily add weight and volume to the system, something that is not desirable in aircraft installations.
Still another difficulty results from the fact that varying systems have various flow rate requirements. Consequently, to meet these variations, it has been necessary to design a combustor unique to each particular system so as to achieve the desired flow rate, as well as reliable ignition. This, of course, increases the costs because of the need for a unique design for each system.
The present invention is directed to overcoming one or more of the above problems.